Group-iii nitride semiconductor laser device, and method of fabricating group-iii nitride semiconductor laser device

ABSTRACT

A group-III nitride semiconductor laser device comprises a laser structure including a support base and a semiconductor region, and an electrode provided on the semiconductor region of the laser structure. The support base comprises a hexagonal group-III nitride semiconductor and has a semipolar primary surface, and the semiconductor region is provided on the semipolar primary surface of the support base. The semiconductor region includes a first cladding layer of a first conductivity type gallium nitride-based semiconductor, a second cladding layer of a second conductivity type gallium nitride-based semiconductor, and an active layer. The first cladding layer, the second cladding layer, and the active layer are arranged along a normal axis to the semipolar primary surface. The active layer comprises a gallium nitride-based semiconductor layer. The c-axis of the hexagonal group-III nitride semiconductor of the support base tilts at a finite angle ALPHA with respect to a normal axis toward an a-axis of the hexagonal group-III nitride semiconductor. The laser structure includes first and second fractured faces intersecting with an a-n plane defined by the normal axis and the a-axis of the hexagonal group-III nitride semiconductor. The laser cavity of the group-III nitride semiconductor laser device includes the first and second fractured faces. The laser structure includes first and second surfaces and the first surface is opposite to the second surface, and each of the first and second fractured faces extends from an edge of the first surface to an edge of the second surface.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a group-III nitride semiconductor laserdevice, and a method of fabricating the group-III nitride semiconductorlaser device.

2. Related Background Art

Non-patent Document 1 describes a semiconductor laser made on a c-planesapphire substrate. The mirror faces of the semiconductor laser areformed by dry etching. The Document 1 shows micrographs of the lasercavity mirror faces of the laser, and describes that the roughness ofthe end faces is about 50 nm.

Non-patent Document 2 describes a semiconductor laser fowled on a(11-22) plane GaN substrate. The mirror faces of the semiconductor laserare formed by dry etching.

Non-patent Document 3 describes a gallium nitride (GaN)-basedsemiconductor laser. It proposes generation of laser light polarized inan off direction of the c-axis of the substrate, in order to use m-planecleaved facets for the laser cavity. Specifically, this Documentdescribes increase of the well thickness on a nonpolar surface anddecrease of the well thickness on a semipolar surface.

Non-patent Document 1: Jpn. J. Appl. Phys. Vol. 35, (1996) L74-L76

Non-patent Document 2: Appl. Phys. Express 1 (2008) 091102

Non-patent Document 3: Jpn. J. Appl. Phys. Vol. 46, (2007) L789

SUMMARY OF THE INVENTION

The band structure of the GaN-based semiconductor has some possibletransitions capable of lasing. According to the Inventor's knowledge, itis considered that in the group-III nitride semiconductor laser deviceusing the semipolar-plane support base the c-axis of which is inclinedtoward the a-axis, the threshold current can be lowered when the laserwaveguide extends along a plane defined by the c-axis and the a-axis.When the laser waveguide extends in this orientation, a mode with thesmallest transition energy (difference between conduction band energyand valence band energy) among the above possible transitions becomescapable of lasing; when this mode becomes capable of lasing, thethreshold current can be reduced.

However, this orientation of the laser waveguide does not allow use ofthe conventional cleaved facets such as c-planes, a-planes, or m-planesfor the laser cavity mirrors. For this reason, the laser cavity mirrorshave been made heretofore by forming dry-etched facets of semiconductorlayers by reactive ion etching (RIB). What is now desired is animprovement in the laser cavity mirrors, which have been formed by RIE,in terms of perpendicularity to the laser waveguide, flatness of thedry-etched facets, or ion damage. It becomes a heavy burden to findprocess conditions for obtaining excellent dry-etched faces in thecurrent technical level.

As far as the inventor knows, in a single group-III nitridesemiconductor laser device formed on the semipolar surface, no one hassucceeded heretofore in achieving both of the laser waveguide, whichextends in the inclination direction (off direction) of the c-axis, andthe end faces for laser cavity mirrors formed without use of dryetching.

The present invention has been accomplished in view of theabove-described circumstances. It is an object of the present inventionto provide a group-III nitride semiconductor laser device with a lasercavity enabling a low threshold current, on the semipolar surface of asupport base that tilts with respect to the c-axis toward the a-axis ofa hexagonal group-III nitride. It is another object of the presentinvention to provide a method of fabricating the group-III nitridesemiconductor laser device.

A group-III nitride semiconductor laser device according to one aspectof the present invention comprises: (a) a laser structure including asupport base comprised of a hexagonal group-III nitride semiconductorand having a semipolar primary surface, and a semiconductor regionprovided on the semipolar primary surface of the support base; and (b)an electrode provided on the semiconductor region of the laserstructure. The semiconductor region comprises a first cladding layer ofa first conductivity type gallium nitride-based semiconductor, a secondcladding layer of a second conductivity type gallium nitride-basedsemiconductor, and an active layer provided between the first claddinglayer and the second cladding layer. The first cladding layer, thesecond cladding layer and the active layer are arranged along a normalaxis to the semipolar primary surface. The active layer comprises agallium nitride-based semiconductor layer. The c-axis of the hexagonalgroup-III nitride semiconductor of the support base tilts at an angleALPHA with respect to the normal axis toward the a-axis of the hexagonalgroup-III nitride semiconductor. The laser structure comprises first andsecond fractured faces which intersect with an a-n plane defined by thea-axis of the hexagonal group-III nitride semiconductor and the normalaxis. A laser cavity of the group-III nitride semiconductor laser devicecomprises the first and second fractured faces. The laser structurecomprises first and second surfaces and the first surface is opposite tothe second surface. Each of the first and second fractured faces extendsfrom an edge of the first surface to an edge of the second surface.

In this group-III nitride semiconductor laser device, since the firstand second fractured faces that form the laser cavity intersect with thea-n plane defined by the normal axis and the a-axis of the hexagonalgroup-III nitride semiconductor, it is feasible to provide the laserwaveguide extending in a direction of an intersecting line between thea-n plane and the semipolar surface. The present invention, therefore,succeeds in providing the group-III nitride semiconductor laser devicewith the laser cavity that enables a low threshold current.

In the group-III nitride semiconductor laser device according to thepresent invention, preferably, the angle between the normal axis and thec-axis of the hexagonal group-III nitride semiconductor falls within therange of not less than 45° and not more than 80° or within the range ofnot less than 100° and not more than 135°.

In this group-III nitride semiconductor laser device, when the angle isin the range of less than 45° and in the range of more than 135°, endfaces made by press are highly likely to be comprised of m-planes. Whenthe angle is in the range of more than 80° and less than 100°, it mightresult in failing to achieve desired flatness and perpendicularity.

In the group-III nitride semiconductor laser device according to thepresent invention, more preferably, the angle between the normal axisand the c-axis of the hexagonal group-III nitride semiconductor fallswithin the range of not less than 59° and not more than 80° or withinthe range of not less than 100° and not more than 121°.

In this group-III nitride semiconductor laser device, when the angle isin the range of not less than 59° and not more than 80° or within therange of not less than 100° and not more than 121°, end faces made bypress are highly likely to be faces nearly perpendicular to the primarysurface of the substrate. When the angle is in the range of more than80° and less than 100°, it may result in failing to achieve desiredflatness and perpendicularity.

In the group-III nitride semiconductor laser device according to thepresent invention, a thickness of the support base is not more than 400μm, which can provide this group-III nitride semiconductor laser devicewith good-quality fractured facets for the laser cavity.

In the group-III nitride semiconductor laser device according to thepresent invention, more preferably, the thickness of the support base isnot less than 50 μm and not more than 100 μm. When the thickness is notless than 50 μm, the handling thereof becomes easier and productionyield becomes higher. When the thickness is not more than 100 μm, it isbetter for obtaining good-quality fractured facets for the laser cavity.

In the group-III nitride semiconductor laser device according to thepresent invention, laser light from the active layer is polarized in adirection of the m-axis of the hexagonal group-III nitridesemiconductor. In this group-III nitride semiconductor laser device, aband transition allowing for achievement of a low threshold current haspolarized nature.

In the group-III nitride semiconductor laser device according to thepresent invention, light in the LED mode in the group-III nitridesemiconductor laser device includes a polarization component I1 and apolarization component I2. The polarization component I1 is in thedirection of the a-axis of the hexagonal group-III nitridesemiconductor, and the polarization component I2 is in a directionindicated by projecting the c-axis of the hexagonal group-III nitridesemiconductor onto the primary surface. The polarization component I1 isgreater than the polarization component I2. This III-nitridesemiconductor laser device can lase with the laser cavity to emit lightin a mode having large emission intensity in the LED mode.

In the group-III nitride semiconductor laser device according to thepresent invention, a surface with a slight tilt in the range of not lessthan −4° and not more than +4° with respect to the semipolar plane ofany one of a {11-22} plane, a {11-21} plane, a {11-2-2} plane, and a{11-2-1} plane, toward an a-plane can be used as the semipolar primarysurface suitably.

This group-III nitride semiconductor laser device allows for provisionof the first and second end faces with flatness and perpendicularityenough to construct the laser cavity of the group-III nitridesemiconductor laser device on the surface slightly tilting from thesetypical semipolar planes.

In the III-nitride semiconductor laser device according to the presentinvention, preferably, the semipolar primary surface is one of a {11-22}plane, a {11-21} plane, a {11-2-2} plane, and a {11-2-1} plane.

This group-III nitride semiconductor laser device allows for provisionof first and second end faces with flatness and perpendicularity enoughto construct the laser cavity of the group-III nitride semiconductorlaser device, on these typical semipolar planes.

In the group-III nitride semiconductor laser device according to thepresent invention, preferably, a stacking fault density of the supportbase is not more than 1×10⁴ cm⁻¹.

In this group-III nitride semiconductor laser device, since the stackingfault density is not more than 1×10⁴ cm⁻¹, the flatness and/orperpendicularity of the fractured facets is unlikely to be disturbed fora certain accidental reason.

In the group-III nitride semiconductor laser device according to thepresent invention, the support base can comprise any one of GaN, AlGaN,AlN, InGaN, and InAlGaN.

In this group-III nitride semiconductor laser device, when the substrateused comprises one of these GaN-based semiconductors, it becomesfeasible to obtain the first and second end faces applicable to thecavity. Use of an AlN substrate or AlGaN substrate allows for increasein the degree of polarization and the enhancement of optical confinementby virtue of low refractive index. Use of an InGaN substrate allows fordecrease in the degree of lattice mismatch between the substrate and thelight emitting layer, and improvement in crystal quality.

The group-III nitride semiconductor laser device according to thepresent invention can further comprise a dielectric multilayer filmprovided on at least one of the first and second fractured faces.

In this group-III nitride semiconductor laser device, an end facecoating is also applicable to the fractured faces. The end face coatallows for adjustment of reflectance.

In the group-III nitride semiconductor laser device according to thepresent invention, the active layer can include a quantum well structureprovided so as to generate light at a wavelength of not less than 360 nmand not more than 600 nm. Since this group-III nitride semiconductorlaser device makes use of the semipolar plane, the resultant group-IIInitride semiconductor laser device makes efficient use of polarizationin the LED mode, and achieves a low threshold current.

In the group-III nitride semiconductor laser device according to thepresent invention, more preferably, the active layer includes a quantumwell structure provided so as to generate light at a wavelength of notless than 430 nm and not more than 550 nm. Since this group-III nitridesemiconductor laser device makes use of the semipolar plane, it allowsfor increase in quantum efficiency through both decrease of thepiezoelectric field and improvement in crystal quality of the lightemitting layer region, and it is thus preferably applicable togeneration of light at the wavelength of not less than 430 nm and notmore than 550 nm.

In the group-III nitride semiconductor laser device according to thepresent invention, an end face of the support base and an end face ofthe semiconductor region are exposed in each of the first and secondfractured faces, and an angle between the end face of the active layerin the semiconductor region and a reference plane perpendicular to thea-axis of the support base of the hexagonal nitride semiconductor is inthe range of not less than (ALPHA−5)° and not more than (ALPHA+5)° in afirst plane defined by the c-axis and the a-axis of the group-IIInitride semiconductor.

This group-III nitride semiconductor laser device has the end faces thatsatisfy the foregoing perpendicularity, concerning the angle taken forone of the c-axis and the a-axis to the other.

In the group-III nitride semiconductor laser device according to thepresent invention, preferably, the angle is in the range of not lessthan −5° and not more than +5° in a second plane perpendicular to thefirst plane and the normal axis.

This group-III nitride semiconductor laser device has the end facessatisfying the foregoing perpendicularity, concerning the angle definedon the plane perpendicular to the normal axis to the semipolar surface.

In the group-III nitride semiconductor laser device according to thepresent invention, the electrode extends in a direction of apredetermined axis, and the first and second fractured faces intersectwith the predetermined axis.

Another aspect of the present invention relates to a method offabricating a group-III nitride semiconductor laser device. This methodcomprises the steps of: (a) preparing a substrate of a hexagonalgroup-III nitride semiconductor and having a semipolar primary surface;(b) forming a substrate product, the substrate product having a laserstructure and the substrate, the laser structure including asemiconductor region, an anode electrode and a cathode electrode, thesemiconductor region being formed on the semipolar primary surface; (c)scribing a first surface of the substrate product in part in a directionof the m-axis of the hexagonal group-III nitride semiconductor; and (d)carrying out breakup of the substrate product by press against a secondsurface of the substrate product, to form another substrate product anda laser bar. The first surface is opposite to the second surface. Thesemiconductor region is located between the first surface and thesubstrate. The laser bar extends from the first surface to the secondsurface, and has first and second end faces formed by the breakup. Thefirst and second end faces constitute a laser cavity of the group-IIInitride semiconductor laser device. The anode electrode and the cathodeelectrode are formed on the laser structure. The semiconductor regioncomprises a first cladding layer of a first conductivity type galliumnitride-based semiconductor, a second cladding layer of a secondconductivity type gallium nitride-based semiconductor, and an activelayer. The active layer is provided between the first cladding layer andthe second cladding layer. The first cladding layer, the second claddinglayer, and the active layer are arranged along a axis normal to thesemipolar primary surface. The active layer comprises a galliumnitride-based semiconductor layer; the c-axis of the hexagonal group-IIInitride semiconductor of the substrate tilts at an angle ALPHA withrespect to the normal axis toward the a-axis of the hexagonal group-IIInitride semiconductor. The first and second end faces intersect with ana-n plane defined by the normal axis and the a-axis of the hexagonalgroup-III nitride semiconductor.

According to this method, the first surface of the substrate product isscribed in the direction of the m-axis of the hexagonal group-IIInitride semiconductor, and thereafter the breakup of the substrateproduct is carried out by press against the second surface of thesubstrate product, thereby forming the other substrate product and thelaser bar. Then, the laser bar is provided with the first and second endfaces that intersect the a-n plane defined by the normal axis and thea-axis of the hexagonal group-III nitride semiconductor. This method offorming end faces provides the first and second end faces with flatnessand perpendicularity enough to construct the laser cavity of thegroup-III nitride semiconductor laser device, or the laser cavity mirrorfaces without ion damage.

In this method, the laser waveguide extends in the direction of tilt ofthe c-axis of the hexagonal group-III nitride, and the mirror end facesof the cavity with which this laser waveguide can be provided are formedwithout use of dry-etched facets.

In the method according to the present invention, forming the substrateproduct comprises performing processing such as slicing or grinding ofthe substrate such that a thickness of the substrate becomes not morethan 400 μm. The second surface can be a processed surface made by theabove processing. Alternatively, it can be a surface including anelectrode formed on the processed surface.

In the method according to the present invention, forming the substrateproduct comprises polishing the substrate such that a thickness of thesubstrate becomes not less than 50 μm and not more than 100 μm. Thesecond surface can be a polished surface formed by the polishing.Alternatively, it can be a surface including an electrode formed on thepolished surface.

When the substrate has such thickness, it is feasible to form the firstand second end faces with flatness and perpendicularity enough toconstruct the laser cavity of the group-III nitride semiconductor laserdevice, or without ion damage, and its production yield is excellent.

In the method according to the present invention, the angle ALPHA fallswithin the range of not less than 45° and not more than 80° and withinthe range of not less than 100° and not more than 135°. When the anglefalls within the range of less than 45° and within the range of morethan 135°, the end faces formed by press are highly likely to becomposed of a-planes. The angle falling within the range of more than80° and less than 100° does not achieve desired flatness andperpendicularity.

In the method according to the present invention, more preferably, theangle ALPHA falls within the range of not less than 59° and not morethan 80° and within the range of not less than 100° and not more than121°. When the angle is in the range of less than 59° and in the rangeof more than 121°, an a-plane is likely to appear in part of an end facemade by press. The angle in the range of more than 80° and less than100° does not achieve desired flatness and perpendicularity.

In the method according to the present invention, a surface with aslight tilt in the range of not less than −4° and not more than +4° fromthe semipolar plane of any one of a {11-22} plane, a {11-21} plane, a{11-2-2} plane, and a {11-2-1} plane, toward the a-plane can be used asthe semipolar primary surface suitably.

When the slight tilt surface tilts from these typical semipolar planes,it is also feasible to provide the first and second end faces withflatness and perpendicularity enough to construct the laser cavity ofthe group-III nitride semiconductor laser device, or without ion damage.

In the method according to the present invention, preferably, thesemipolar primary surface is made of any one of a {11-22} plane, a{11-21} plane, a {11-2-2} plane, and a {11-2-1} plane.

These semipolar planes can provide the first and second end faces withflatness and perpendicularity enough to construct the laser cavity ofthe group-III nitride semiconductor laser device, or without ion damage.

In the method according to the present invention, the scribing isperformed using a laser scriber, the scribing forms a scribed groove,and a length of the scribed groove is shorter than a length of anintersecting line between an m-n plane defined by the normal axis andthe m-axis of the hexagonal group-III nitride semiconductor, and thefirst surface.

According to this method, the other substrate product and the laser barare formed by fracture of the substrate product. This fracture isbrought about by using the scribed groove shorter than a fracture lineof the laser bar.

In the method according to the present invention, an end face of theactive layer in each of the first and second end faces can make an anglein the range of not less than (ALPHA−5)° and not more than (ALPHA+5)° ina plane defined by the c-axis and the a-axis of the hexagonal group-IIInitride semiconductor, and the angle is defined with respect to areference plane perpendicular to the a-axis of the support base of thehexagonal nitride semiconductor.

This method allows for forming the end faces with the above-mentionedperpendicularity for the angle taken from one of the c-axis and thea-axis to the other.

In the method according to the present invention, the substrate cancomprise any one of GaN, AlN, AlGaN, InGaN and InAlGaN.

This method allows the first and second end faces applicable to thecavity obtained through the use of the substrate made of one of theseGaN-based semiconductors.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objects and the other objects, features, and advantages of thepresent invention can more readily become apparent in view of thefollowing detailed description of the preferred embodiments of thepresent invention proceeding with reference to the accompanyingdrawings.

FIG. 1 is a drawing schematically showing a structure of a group-IIInitride semiconductor laser device according to an embodiment of thepresent invention.

FIG. 2 is a drawing showing a band structure in an active layer in thegroup-III nitride semiconductor laser device.

FIG. 3 is a drawing showing polarization of emission in the active layerof the group-III nitride semiconductor laser device.

FIG. 4 is a drawing showing a relation between an end face of thegroup-III nitride semiconductor laser device and an a-plane of theactive layer.

FIG. 5 is a flowchart showing major steps in a method of fabricating thegroup-III nitride semiconductor laser device according to theembodiment.

FIG. 6 is a drawing schematically showing major steps in the method offabricating the group-III nitride semiconductor laser device accordingto the embodiment.

FIG. 7 is a drawing showing a scanning electron microscope image of acavity end face, along with a {11-21} plane in crystal lattices.

FIG. 8 is a drawing showing a structure of a laser diode shown inExample 1.

FIG. 9 is a drawing showing a relation of determined polarization degreeρ versus threshold current density.

FIG. 10 is a drawing showing a relation of tilt angles of the c-axistoward the a-axis of GaN substrate versus lasing yield.

FIG. 11 is a drawing showing a relation of stacking fault density versuslasing yield.

FIG. 12 is a drawing showing a relation of substrate thickness versuslasing yield.

FIG. 13 is a drawing showing angles between (11-21) plane and otherplane orientations (indices).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The expertise of the present invention can be readily understood in viewof the following detailed description with reference to the accompanyingdrawings provided by way of illustration only.

The following will describe embodiments of the group-III nitridesemiconductor laser device and the method of fabricating the group-IIInitride semiconductor laser device according to the present invention,with reference to the accompanying drawings. The same portions will bedenoted by the same reference symbols if possible.

FIG. 1 is a drawing schematically showing a structure of a group-IIInitride semiconductor laser device according to an embodiment of thepresent invention. The group-III nitride semiconductor laser device 11has a gain-guiding type structure, but embodiments of the presentinvention are not limited to the gain-guiding type structure. Thegroup-III nitride semiconductor laser device 11 has a laser structure 13and an electrode 15. The laser structure 13 includes a support base 17and a semiconductor region 19. The support base 17 comprises a hexagonalgroup-III nitride semiconductor and has a semipolar primary surface 17 aand a back surface 17 b. The semiconductor region 19 is provided on thesemipolar primary surface 17 a of the support base 17. The electrode 15is provided on the semiconductor region 19 of the laser structure 13.The semiconductor region 19 includes a first cladding layer 21, a secondcladding layer 23 and an active layer 25. The first cladding layer 21comprises a first conductivity type gallium nitride based semiconductor,e.g., n-type AlGaN, n-type InAlGaN, or the like. The second claddinglayer 23 comprises a second conductivity type GaN-based semiconductor,e.g., p-type AlGaN, p-type InAlGaN, or the like. The active layer 25 isprovided between the first cladding layer 21 and the second claddinglayer 23. The active layer 25 includes gallium nitride basedsemiconductor layers, and the gallium nitride based semiconductor layersare, for example, well layers 25 a. The active layer 25 includes barrierlayers 25 b of a gallium nitride based semiconductor, and the welllayers 25 a and the barrier layers 25 b are alternately arranged. Thewell layers 25 a comprise, for example, of InGaN or the like and thebarrier layers 25 b, for example, GaN, InGaN, or the like. The activelayer 25 can include a quantum well structure provided so as to generatelight at the wavelength of not less than 360 nm and not more than 600nm, and making use of the semipolar surface is preferably applicable togeneration of light at the wavelength of not less than 430 nm and notmore than 550 nm. The first cladding layer 21, the second cladding layer23, and the active layer 25 are arranged along an axis NX normal to thesemipolar primary surface 17 a. In the group-III nitride semiconductorlaser device 11, the laser structure 13 includes a first fractured face27 and a second fractured face 29, which intersect with an a-n planedefined by the normal axis NX and the a-axis of the hexagonal group-IIInitride semiconductor.

Referring to FIG. 1, an orthogonal coordinate system S and a crystalcoordinate system CR are depicted. The normal axis NX is directed alonga direction of the Z-axis of the orthogonal coordinate system S. Thesemipolar primary surface 17 a extends in parallel with a predeterminedplane defined by the X-axis and the Y-axis of the orthogonal coordinatesystem S. In FIG. 1, a typical c-plane Sc is also depicted. The c-axisof the hexagonal group-III nitride semiconductor of the support base 17tilts at an angle ALPHA with respect to the normal axis NX toward thea-axis of the hexagonal group-III nitride semiconductor.

The group-III nitride semiconductor laser device 11 further has aninsulating film 31. The insulating film 31 covers a surface 19 a of thesemiconductor region 19 of the laser structure 13, and the semiconductorregion 19 is located between the insulating film 31 and the support base17. The support base 17 comprises a hexagonal group-III nitridesemiconductor. The insulating film 31 has an opening 31 a, and theopening 31 a extends in a direction of an intersecting line LIX betweenthe surface 19 a of the semiconductor region 19 and the foregoing a-nplane, and has, for example, a stripe shape. The electrode 15 is incontact with the surface 19 a of the semiconductor region 19 (e.g., acontact layer 33 of the second conductivity type) through the opening 31a, and extends in the direction of the foregoing intersecting line LIX.In the group-III nitride semiconductor laser device 11, a laserwaveguide includes the first cladding layer 21, the second claddinglayer 23 and the active layer 25, and extends in the direction of theforegoing intersecting line LIX.

In the group-III nitride semiconductor laser device 11, the firstfractured face 27 and the second fractured face 29 intersect with thea-n plane defined by the a-axis of the hexagonal group-III nitridesemiconductor and the normal axis NX. A laser cavity of the group-IIInitride semiconductor laser device 11 includes the first and secondfractured faces 27 and 29, and the laser waveguide extends from one ofthe first fractured face 27 and the second fractured face 29 to theother. The laser structure 13 includes a first surface 13 a and a secondsurface 13 b, and the first surface 13 a is opposite to the secondsurface 13 b. The first and second fractured faces 27, 29 extend from anedge 13 c of the first surface 13 a to an edge 13 d of the secondsurface 13 b. The first and second fractured faces 27, 29 are differentfrom the conventional cleaved facets like c-planes, m-planes, ora-planes.

In this group-III nitride semiconductor laser device 11, the first andsecond fractured faces 27, 29 that form the laser cavity intersect withthe a-n plane. This allows for provision of the laser waveguideextending in the direction of the intersecting line between the a-nplane and the semipolar surface 17 a. For this reason, the group-IIInitride semiconductor laser device 11 has the laser cavity enabling alow threshold current.

The group-III nitride semiconductor laser device 11 includes an n-sideoptical guide layer 35 and a p-side optical guide layer 37. The n-sideoptical guide layer 35 includes a first portion 35 a and a secondportion 35 b, and the n-side optical guide layer 35 comprises, forexample, of GaN, InGaN, or the like. The p-side optical guide layer 37includes a first portion 37 a and a second portion 37 b, and the p-sideoptical guide layer 37 comprises, for example, of GaN, InGaN, or thelike. A carrier block layer 39 is provided, for example, between thefirst portion 37 a and the second portion 37 b. Another electrode 41 isprovided on the back surface 17 b of the support base 17, and theelectrode 41 covers, for example, the back surface 17 b of the supportbase 17.

FIG. 2 is a drawing showing a band structure in the active layer in thegroup-III nitride semiconductor laser device. FIG. 3 is a drawingshowing polarization of emission from the active layer 25 of thegroup-III nitride semiconductor laser device 11. FIG. 4 is a schematiccross sectional view taken along a plane defined by the c-axis and thea-axis. With reference to Part (a) of FIG. 2, three possible transitionsbetween the conduction band and valence bands in the vicinity of F pointof the band structure BAND are shown. The energy difference between bandA and band B is relatively small. An emission by transition Ea betweenthe conduction band and band A is polarized in the m-axis direction, andan emission by transition Eb between the conduction band and band B ispolarized in a direction of the c-axis projected onto the primarysurface. Concerning lasing, a threshold of transition Ea is smaller thana threshold of transition Eb.

With reference to Part (b) of FIG. 2, there are shown spectra of lightin the LED mode in the group-III nitride semiconductor laser device 11.The light in the LED mode includes a polarization component I1 in thedirection of the m-axis of the hexagonal group-III nitridesemiconductor, and a polarization component I2 in the direction of theprojected c-axis of the hexagonal group-III nitride semiconductor ontothe primary surface, and the polarization component I1 is larger thanthe polarization component I2. Degree of polarization p is defined by(I1−I2)/(I1+I2). The laser cavity of the group-III nitride semiconductorlaser device 11 enables the device to emit a laser beam in the mode thathas large emission intensity in the LED mode.

As shown in FIG. 3, the device may be further provided with dielectricmultilayer film 43 a, 43 b on at least one of the first and secondfractured faces 27, 29 or on the respective faces. An end face coatingis also applicable to the fractured faces 27, 29. The end face coatingallows adjustment of their reflectance.

As shown in Part (b) of FIG. 3, the laser light L from the active layer25 is polarized in the direction of the m-axis of the hexagonalgroup-III nitride semiconductor. In this group-III nitride semiconductorlaser device 11, a band transition allowing for implementation of a lowthreshold current has polarized nature. The first and second fracturedfaces 27, 29 for the laser cavity are different from the conventionalcleaved facets like c-planes, m-planes, or a-planes. But, the first andsecond fractured faces 27, 29 have flatness and perpendicularity asmirrors for lasing cavity. For this reason, by using the first andsecond fractured faces 27, 29 and the laser waveguide extending betweenthese fractured faces 27, 29, as shown in Part (b) of FIG. 3, it becomesfeasible to achieve low-threshold lasing through the use of the emissionby transition Ea stronger than the emission by transition Eb that ispolarized in the direction indicated by the c-axis projected onto theprimary surface.

In the group-III nitride semiconductor laser device 11, an end face 17 cof the support base 17 and an end face 19 c of the semiconductor region19 are exposed in each of the first and second fractured faces 27 and29, and the end face 17 c and the end face 19 c are covered with thedielectric multilayer film 43 a. An angle BETA between an a-axis vectorAA of the active layer 25 and a vector NA normal to the end face 17 c ofthe support base 17 and an end face 25 c in the active layer 25 has acomponent (BETA)₁ defined on a first plane S1, which is defined by thec-axis and a-axis of the group-III nitride semiconductor, and acomponent (BETA)₂ defined on a second plane S2 (which is referred to as“S2” in FIG. 3), which is perpendicular to the first plane S1 (which isreferred to as “S1” in FIG. 3) and the normal axis NX. The component(BETA)₁ is preferably in the range of not less than (ALPHA−5)° and notmore than (ALPHA+5)° in the first plane S1 defined by the c-axis anda-axis of the group-III nitride semiconductor. This angular range isshown as an angle between a typical a-plane S_(A) and a reference planeF_(M) in FIG. 4. The typical a-plane S_(A) is depicted from the insideto the outside of the laser structure in FIG. 4, for easierunderstanding. The reference plane F_(M) extends along the end face 25 cof the active layer 25. This group-III nitride semiconductor laserdevice 11 has the end faces in which the angle BETA taken from one ofthe c-axis and the a-axis to the other satisfies the aforementionedperpendicularity. The component (BETA)₂ is preferably in the range ofnot less than −5° and not more than +5° on the second plane S2. Here,BETA²=(BETA)₁ ²+(BETA)₂ ². The end faces 27 and 29 of the group-IIInitride semiconductor laser device 11 satisfy the aforementionedperpendicularity as to the in-plane angle defined in the plane that isperpendicular to the normal axis NX to the semipolar surface 17 a.

Referring again to FIG. 1, in the group-III nitride semiconductor laserdevice 11, the thickness DSUB of the support base 17 is preferably notmore than 400 μm. Preferably, this group-III nitride semiconductor laserdevice can provide good-quality fractured faces for the laser cavity. Inthe group-III nitride semiconductor laser device 11, the thickness DSUBof the support base 17 is more preferably not less than 50 μm and notmore than 100 μm. This group-III nitride semiconductor laser device 11can be provided good-quality fractured faces more preferred for thelaser cavity. Furthermore, its handling becomes easier and theproduction yield can be improved.

In the group-III nitride semiconductor laser device 11, the angle ALPHAbetween the normal axis NX and the c-axis of the hexagonal group-IIInitride semiconductor is preferably not less than 45° and preferably notmore than 80°, and the angle ALPHA is preferably not less than 100° andpreferably not more than 135°. When the angle is in the range of lessthan 45° and in the range of more than 135°, the end faces made by pressare highly likely to be comprised of a-planes. When the angle is in therange of more than 80° and less than 100°, it could result in failing toachieve desired flatness and perpendicularity.

In the group-III nitride semiconductor laser device 11, more preferably,the angle ALPHA between the normal axis NX and the c-axis of thehexagonal group-III nitride semiconductor is not less than 59° and notmore than 80°. Furthermore, the angle ALPHA is particularly preferablynot less than 100° and not more than 121°. When the angle is in therange of less than 59° and in the range of more than 121°, an a-planecan be formed in part of an end face made by press. When the angle is inthe range of more than 80° and less than 100°, it could result infailing to achieve desired flatness and perpendicularity.

The semipolar primary surface 17 a can be any one of a {11-22} plane, a{11-21} plane, a {11-2-2} plane and a {11-2-1} plane. Furthermore, asurface with a slight tilt in the range of not less than −4° and notmore than +4° with respect to these planes may also be applied as theprimary surface. On the semipolar surface 17 a of one of these typicalplanes, it is feasible to provide the first and second end faces 27 and29 with flatness and perpendicularity enough to construct the lasercavity of the group-III nitride semiconductor laser device 11.Furthermore, end faces with sufficient flatness and perpendicularity areobtained in an angular range across these typical plane orientations.

In the group-III nitride semiconductor laser device 11, the stackingfault density of the support base 17 can be not more than 1×10⁴ cm⁻¹.Since the stacking fault density is not more than 1×10^(4 cm) ⁻¹, theflatness and/or perpendicularity of the fractured faces is less likelyto be disturbed for a certain accidental reason. The support base 17 cancomprise any one of GaN, AlN, AlGaN, InGaN, and InAlGaN. When thesubstrate of any one of these GaN-based semiconductors is used, the endfaces 27 and 29 applicable to the cavity can be obtained. When an AlN orAlGaN substrate is used, it is feasible to increase the degree ofpolarization and to enhance optical confinement by virtue of lowrefractive index. When an InGaN substrate is used, it is feasible todecrease degree of the lattice mismatch between the substrate and thelight emitting layer and to improve crystal quality.

FIG. 5 is a drawing showing major steps in a method of fabricating thegroup-III nitride semiconductor laser device according to the presentembodiment. With reference to Part (a) of FIG. 6, a substrate 51 isshown. In step S101, the substrate 51 is prepared for fabrication of thegroup-III nitride semiconductor laser device. The c-axis (vector VC) ofthe hexagonal group-III nitride semiconductor of the substrate 51 tiltsat an angle ALPHA with respect to the normal axis NX toward the a-axis(vector VA) of the hexagonal group-III nitride semiconductor.Accordingly, the substrate 51 has a semipolar primary surface 51 a ofthe hexagonal group-III nitride semiconductor.

In step S102, a substrate product SP is formed. In Part (a) of FIG. 6,the substrate product SP is depicted as a member of a nearly disklikeshape, but the shape of the substrate product SP is not limited thereto.For obtaining the substrate product SP, step S103 is first performed toform a laser structure 55. The laser structure 55 includes asemiconductor region 53 and the substrate 51, and in step S103, thesemiconductor region 53 is grown on the semipolar primary surface 51 a.For forming the semiconductor region 53, a first conductivity typeGaN-based semiconductor region 57, a light emitting layer 59, and asecond conductivity type GaN-based semiconductor region 61 are grownsequentially on the semipolar primary surface 51 a. The GaN-basedsemiconductor region 57 can include, for example, an n-type claddinglayer and the GaN-based semiconductor region 61 can include, forexample, a p-type cladding layer. The light emitting layer 59 isprovided between the GaN-based semiconductor region 57 and the GaN-basedsemiconductor region 61, and can include an active layer, optical guidelayers, an electron block layer, and so on. The GaN-based semiconductorregion 57, the light emitting layer 59, and the second conductivity typeGaN-based semiconductor region 61 are arranged in the direction of thenormal axis NX to the semipolar primary surface 51 a. Thesesemiconductor layers are epitaxially grown thereon. The surface of thesemiconductor region 53 is covered with an insulating film 54. Theinsulating film 54 comprises, for example, of silicon oxide. Theinsulating film 54 has an opening 54 a. The opening 54 a has, forexample, a stripe shape.

Step S104 is carried out to form an anode electrode 58 a and a cathodeelectrode 58 b on the laser structure 55. Before forming the electrodeon the back surface of the substrate 51, the back surface of thesubstrate used in crystal growth is polished to form a substrate productSP in desired thickness DSUB. In formation of the electrodes, forexample, the anode electrode 58 a is formed on the semiconductor region53, and the cathode electrode 58 b is formed on the back surface(polished surface) 51 b of the substrate 51. The anode electrode 58 aextends in the X-axis direction, and the cathode electrode 58 b coversthe entire area of the back surface 51 b. After these steps, thesubstrate product SP is obtained. The substrate product SP includes afirst surface 63 a, and a second surface 63 b located opposite thereto.The semiconductor region 53 is located between the first surface 63 aand the substrate 51.

Step S105 is carried out, as shown in Part (b) of FIG. 6, to scribe thefirst surface 63 a of the substrate product SP. This scribing step iscarried out with a laser scriber 10 a. This scribing step forms scribedgrooves 65 a. In Part (b) of FIG. 6, five scribed grooves are alreadyformed, and formation of a scribed groove 65 b is in progress with laserbeam LB. The length of the scribed grooves 65 a is shorter than thelength of an intersecting line AIS between the first surface 63 a and anm-n plane defined by the normal axis NX and the m-axis of the hexagonalgroup-III nitride semiconductor, and the laser beam LB is applied to apart of the intersecting line MS. By the application with the laser beamLB, grooves extending in the specific direction and reaching thesemiconductor region are formed in the first surface 63 a. The scribedgrooves 65 a can be formed, for example, in an edge of the substrateproduct SP.

As shown in Part (c) of FIG. 6, step S106 is carried out to implementbreakup of the substrate product SP by press against the second surface63 b of the substrate product SP, thereby forming a substrate productSP1 and a laser bar LB1. The press is carried out with a breakingdevice, such as, a blade 69. The blade 69 includes an edge 69 aextending in one direction, and at least two blade faces 69 b and 69 cthat are formed to define the edge 69 a. The pressing onto the substrateproduct SP1 is carried out on a support device 71. The support device 71includes a support table 71 a and a recess 71 b, and the recess 71 bextends in one direction. The recess 71 b is formed in the support table71 a. The orientation and position of the scribed groove 65 a of thesubstrate product SP1 are aligned with the extending direction of therecess 71 b of the support device 71 to position the substrate productSP1 to the recess 71 b on the support device 71. The orientation of theedge of the breaking device is aligned with the extending direction ofthe recess 71 b, and the edge of the breaking device is pressed againstthe substrate product SP1 from a direction intersecting with the secondsurface 63 b. The intersecting direction is preferably an approximatelyvertical direction to the second surface 63 b. This implements thebreakup of the substrate product SP to form the substrate product SP1and laser bar LB1. The press results in forming the laser bar LB1 withfirst and second end faces 67 a and 67 b, and these end faces 67 a and67 b have perpendicularity and flatness enough to make at least a partof the light emitting layer applicable to mirrors for the laser cavityof the semiconductor laser.

The laser bar LB1 thus formed has the first and second end faces 67 a,67 b formed by the aforementioned breakup, and each of the end faces 67a, 67 b extends from the first surface 63 a to the second surface 63 b.The end faces 67 a, 67 b form the laser cavity of the group-III nitridesemiconductor laser device, and intersect with the XZ plane. This XZplane corresponds to the m-n plane defined by the normal axis NX and thea-axis of the hexagonal group-III nitride semiconductor.

By use of this method, the first surface 63 a of the substrate productSP is scribed in the direction of the m-axis of the hexagonal group-IIInitride semiconductor, and thereafter the breakup of the substrateproduct SP is carried out by press against the second surface 63 b ofthe substrate product SP, thereby forming the new substrate product SP1and the laser bar LB1. This method allows the formation of the first andsecond end faces 67 a, 67 b, which intersect with the a-n plane, in thelaser bar LB1. This end face forming method provides the first andsecond end faces 67 a, 67 b with flatness and perpendicularity enough toconstruct the laser cavity of the group-III nitride semiconductor laserdevice.

In this method, the laser waveguide thus formed extends in the directionof inclination of the c-axis of the hexagonal group-III nitride. The endfaces of the laser cavity mirror allowing for provision of this laserwaveguide are formed without use of dry-etching.

This method involves the fracturing of the substrate product SP1,thereby forming the new substrate product SP1 and the laser bar LB1. InStep S107, the breakup is repeatedly carried out by press to produce anumber of laser bars. This fracture propagates along the scribed grooves65 a shorter than a fracture line BREAK of the laser bar LB1.

In Step S108, dielectric multilayer films is formed on the end faces 67a, 67 b of the laser bar LB1 to form a laser bar product. In Step S109,this laser bar product is separated into individual semiconductor laserdies.

In the fabrication method according to the present embodiment, the angleALPHA can be in the range of not less than 45° and not more than 80° andin the range of not less than 100° and not more than 135°. When theangle is in the range of less than 45° and in the range of more than135°, the end face made by press becomes highly likely to be comprisedof an a-plane. When the angle is in the range of more than 80° and lessthan 100°, it may result in failing to achieve desired flatness andperpendicularity. More preferably, the angle ALPHA can be in the rangeof not less than 59° and not more than 80° and in the range of not lessthan 100° and not more than 121°. When the angle is in the range of lessthan 59° and in the range of more than 121°, an a-plane can be formed inpart of an end face formed by press. When the angle is in the range ofmore than 80° and less than 100°, it may result in failing to achievedesired flatness and perpendicularity. The semipolar primary surface 51a can be any one of a {11-22} plane, a {11-21} plane, a {11-2-2} plane,and a {11-2-1} plane. Furthermore, a surface slightly tilted in therange of not less than −4° and not more than +4° from the above planesis also used as the primary surface. On these typical semipolar planes,it is feasible to provide the end faces for the laser cavity withflatness and perpendicularity enough to construct the laser cavity ofthe group-III nitride semiconductor laser device.

The substrate 51 can be made of any one of GaN, AlN, AlGaN, InGaN, andInAlGaN. When any one of these GaN-based semiconductors is used for thesubstrate, it is feasible to obtain the end faces applicable to thelaser cavity. The substrate 51 is preferably made of GaN.

In the step S104 of forming the substrate product SP, the semiconductorsubstrate used in crystal growth can be one subjected to processing suchas slicing or grinding so that the substrate thickness becomes not morethan 400 μm, whereby the second surface 63 b of the semiconductorsubstrate becomes a processed surface formed by polishing. In thissubstrate thickness, the end faces 67 a, 67 b can be formed in goodyield, and are provided with flatness and perpendicularity enough toconstruct the laser cavity of the group-III nitride semiconductor laserdevice or without ion damage. More preferably, the second surface 63 bcan be is a polished surface formed by polishing, and the thickness ofthe polished substrate is not more than 100 μm. For facilitating tohandle the substrate product SP, the substrate thickness is preferablynot less than 50 μm.

In the production method of the laser end faces according to the presentembodiment, the angle BETA explained with reference to FIG. 3 can bealso defined in the laser bar LB1. In the laser bar LB1, the component(BETA)₁ of the angle BETA is preferably in the range of not less than(ALPHA−5)° and not more than (ALPHA+5)° on a first plane (planecorresponding to the first plane S1 in the description with reference toFIG. 3) defined by the c-axis and a-axis of the group-III nitridesemiconductor. The end faces 67 a, 67 b of the laser bar LB1 satisfy theaforementioned perpendicularity as to the angle component of the angleBETA taken from one of the c-axis and the a-axis to the other. Thecomponent (BETA)₂ of the angle BETA is preferably in the range of notless than −5° and not more than +5° on a second plane (planecorresponding to the second plane S2 shown in FIG. 3). These end faces67 a, 67 b of the laser bar LB1 also satisfy the aforementionedperpendicularity as to the angle component of the angle BETA defined onthe plane perpendicular to the normal axis NX to the semipolar surface51 a.

The end faces 67 a, 67 b are formed by break by press against theplurality of GaN-based semiconductor layers epitaxially grown on thesemipolar surface 51 a. Since they are epitaxial films on the semipolarsurface 51 a, each of the end faces 67 a, 67 b are not cleaved facetseach having a low plane index like c-planes, m-planes, or a-planes whichhave been used heretofore for the conventional laser cavity mirrors.However, through the break of the stack of epitaxial films on thesemipolar surface 51 a, the end faces 67 a, 67 b have flatness andperpendicularity applicable as laser cavity mirrors.

Example 1

A GaN substrate with a semipolar surface is prepared, andperpendicularity of a fractured facet is observed as described below.The above substrate used has a {11-21}-plane GaN substrate formed bycutting a (0001) GaN ingot, thickly grown by HYPE, at the angle of 73°toward the a-axis. The primary surface of the GaN substrate ismirror-finished, and the back surface has pear-skin which is finished bygrinding. The thickness of the substrate is 370 μm.

On the back side in the pear-skin finish, a marking line is drawn, witha diamond pen, perpendicularly to the direction of the c-axis projectedon the primary surface of the substrate, and thereafter the substrate isfractured by press. For observing the perpendicularity of the resultantfractured face, the substrate is observed from the m-plane directionwith a scanning electron microscope.

Part (a) of FIG. 7 shows a scanning electron microscope image of thefractured face observed from the m-plane direction, and the fracturedface is shown as the right end face. As seen from the image, a part ofthe fractured face has flatness and perpendicularity to the semipolarprimary surface. The laser diode can be provided with the end faceshaving the improved flatness and perpendicularity, which is achieved bythinning the thickness of the substrate by polishing to improve thescribing method.

Example 2

It is found in Example 1 that in the GaN substrate having the semipolar{11-21} plane, the fractured face is obtained by pressing the substrateafter drawing the marking line perpendicular to the projected directionof the c-axis onto the primary surface of the substrate, and has theflatness and perpendicularity to the primary surface of the substrate.For estimating applicability of this fractured face to the laser cavity,a laser diode shown in FIG. 8 is grown by organometallic vapor phaseepitaxy as described below. The raw materials used are as follows:trimethyl gallium (TMGa); trimethyl aluminum (TMA1); trimethyl indium(TMIn); ammonia (NH₃); and silane (SiH₄). A substrate 71 is prepared. AGaN substrate is prepared as the substrate 71, and the GaN substrate iscut with a wafer slicing apparatus at an angle in the range of 0° to 90°to the m-axis from a (0001) GaN ingot thickly grown by HVPE, in such amanner that the angle ALPHA of the c-axis tilted toward the m-axis has adesired off angle in the range of 0° to 90°. For example, when thesubstrate is formed by cutting at the angle of 75°, the resultantsubstrate is prepared as a GaN substrate having a {11-21}-plane and itis represented by reference symbol 71 a in the hexagonal crystal latticeshown in Part (b) of FIG. 7.

Before the growth, the substrate is observed by the cathodoluminescencemethod in order to estimate the stacking fault density of the substrate.In the cathodoluminescence, an emission process of carriers excited byan electron beam is observed and in a stacking fault, non-radiativerecombination of carriers occurs in the vicinity thereof, so that thestacking fault is expected be observed as a dark line. The stackingfault density is defined as a density (line density) per unit length ofdark lines observed. The cathodoluminescence method of nondestructivemeasurement is applied herein in order to estimate the stacking faultdensity, but it is also possible to use destructive measurement, such asa transmission electron microscope. When a cross section of a sample isobserved from the m-axis direction with the transmission electronmicroscope, a defect extending in the a-axis direction from thesubstrate toward the sample surface indicates a stacking fault containedin the support base, and the line density of stacking faults can bedetermined in the same manner as in the cathodoluminescence method.

The above substrate 71 is placed on a susceptor in a reactor, and theepitaxial layers are grown in the following growth procedure. First, ann-type GaN 72 is grown thereon and its the thickness is 1000 nm. Next,an n-type InAlGaN cladding layer 73 is grown thereon and its thicknessis 1200 nm. Thereafter, an n-type GaN guide layer 74 a and an undopedInGaN guide layer 74 b are grown, their thickness are 200 nm and 65 nm,respectively, and then a three-cycle MQW 75 constituted by GaN 15 nmthick/InGaN 3 nm thick is grown thereon. Subsequently grown thereon arean undoped InGaN guide layer 76 a of the thickness of 65 nm, a p-typeAlGaN block layer 77 a of the thickness of 20 nm, and a p-type GaN guidelayer 76 b of the thickness of 200 nm. Then, a p-type InAlGaN claddinglayer 77 b is grown thereon, and its thickness is 400 nm. Finally, ap-type GaN contact layer 78 is grown thereon and its thickness is 50 nm.

An insulating film 79 of SiO₂ is deposited on the contact layer 78 andthen photolithography and wet etching processes are applied to form astripe window having the width of 10 μm in the insulating film 79. Inthis step, two types of contact windows are formed in two stripedirections, respectively. These laser stripes are formed in thefollowing directions: (1) A-direction (direction of the contact windowextending along the predetermined plane defined by the c-axis and thea-axis); and (2) M-direction: <1-100> direction.

After the formation of the stripe window, a p-side electrode 80 a ofNi/Au and a pad electrode of Ti/Al are made by vapor deposition. Next,the back surface of the GaN substrate (GaN wafer) is polished usingdiamond slurry to produce a substrate product with the mirror-polishedback surface. Then, the thickness of the thus formed substrate productis measured with a contact film thickness meter. The measurement ofsubstrate thickness may also be carried out with a microscope from theobservation of a cross section of a prepared sample. The microscopeapplicable herein can be an optical microscope or a scanning electronmicroscope. An n-side electrode 80 b of Ti/Al/Ti/Au is formed by vapordeposition on the back surface (polished surface) of the GaN substrate(GaN wafer).

The laser cavity mirrors for these two types of laser stripes areproduced with a laser scriber that uses the YAG laser at the wavelengthof 355 nm. When the break is implemented with the laser scriber, theyield defined by lasing operation can be improved as compared with breakimplemented using a diamond scriber. The conditions for formation of thescribed grooves are as follows: laser beam output power of 100 mW;scanning speed of 5 mm/s. The scribed grooves thus formed each has, forexample, the length of 30 μm, the width of 10 μm, and the depth of 40μm. The scribed grooves are formed by applying the laser beam throughthe aperture of the insulating film of the substrate directly to theepitaxially grown surface at the pitch of 800 μm. The cavity length is600 μm.

The laser cavity mirrors are made through fracture by use of a blade. Alaser bar is produced by break by press against the back side of thesubstrate. More specifically, Parts (b) and (c) of FIG. 7 show relationsbetween crystal orientations and fractured faces, for the {11-21}-planeGaN substrate. Part (b) of FIG. 7 shows the laser stripe that isprovided to extend (1) in the M-direction, and shows end faces 81 a, 81b for the laser cavity along with the semipolar surface 71 a. The endfaces 81 a, 81 b are approximately perpendicular to the semipolarsurface 71 a, but are different from the conventional cleaved facetslike the hitherto used c-planes, m-planes, or a-planes. Part (c) of FIG.7 shows the laser stripe that is provided to extend (2) in the <11-20>direction, and shows end faces 81 c, 81 d for the laser cavity alongwith the semipolar surface 71 a. The end faces 81 c, 81 d areapproximately perpendicular to the semipolar surface 71 a and arecomposed of m-planes.

The fractured faces made by break are observed with a scanning electronmicroscope, and no prominent unevenness is observed in each of (1) and(2). From this result, the flatness (magnitude of unevenness) of thefractured faces can be not more than 20 nm. Furthermore, theperpendicularity of the fractured faces to the surface of the sample canbe within the range of ±5°.

The end faces of the laser bar are coated with a dielectric multilayerfilm by vacuum vapor deposition. The dielectric multilayer film iscomposed of an alternate stack of SiO₂ and TiO₂. Each thickness thereofis adjusted in the range of 50 to 100 nm and is designed so that thecenter wavelength of reflectance falls within the range of 500 to 530nm. The reflecting surface on one side has an alternate stack of tencycles and a designed value of reflectance of about 95%, and thereflecting surface on the other side has an alternate stack of sixcycles and a designed value of reflectance of about 80%.

The devices thus formed are energized to make their evaluation at roomtemperature. A pulsed power source is used as a power supply for theenergization, and supplies pulses with the pulse width of 500 ns and theduty ratio of 0.1%, and the energization is implemented through probingneedles that are in contact with the surface electrodes. In light outputmeasurement, an emission from the end face of the laser bar is detectedwith a photodiode to obtain a current-light output characteristic (I-Lcharacteristic). In measurement of emission wavelength, the emissionfrom the end face of the laser bar is supplied through an optical fiberto a spectrum analyzer of a detector to measure a spectrum thereof. Inestimation of a polarization, the emission from the laser bar is made topass through a polarizing plate by rotation, thereby determining thepolarization state. In observation of LED-mode emission, an opticalfiber is aligned to the front surface side of the laser bar to measureoptical emission from the front surface.

The polarization in the laser beam is measured for every laser device,and it is found that the laser beam is polarized in the m-axisdirection. The lasing wavelength is in a range of 500-530 nm.

The polarization state in the LED mode (i.e., spontaneous emission) ismeasured for every laser device. When the polarization component in them-axis direction is referred to as I1, and the polarization component inthe direction of the projected m-axis onto the primary surface isreferred to as I2, the polarization degree ρ is defined as(I1−I2)/(I1+I2). The relation between determined polarization degree ρand minimum of threshold current density is investigated, and the resultobtained is shown in FIG. 9. As seen from FIG. 9, the threshold currentdensity demonstrates a significant decrease in the laser (1) with thelaser stripe along the A-direction when the polarization degree ispositive. Namely, it is seen that when the polarization degree ispositive (I1>I2) and the waveguide is provided along an off direction,the threshold current density is significantly decreased. The data shownin FIG. 9 is as follows.

Polarization Threshold current, Threshold current, degree, (A-directionstripe), (<11-20> stripe), −0.15,    5, 30; 0.05, 30, 45; 0.23, 20, 60;0.35, 15.

The relation between the tilt angle of the c-axis of the GaN substratetoward the a-axis, and lasing yield is investigated, and the result thusobtained is shown in FIG. 10. In the present example, the lasing yieldis defined as (the number of oscillating chips)/(the number of measuredchips). FIG. 10 is a plot for substrates, having the stacking faultdensity of substrate of not more than 1×10⁴ (cm⁻¹), on which lasers withthe laser stripe along (1) the A-direction are formed. As seen from FIG.10, the lasing yield is extremely low in the off angles of not more than45°. The observation of the end faces with an optical microscope findsthat an m-plane is formed in almost all chips at the tilt angles smallerthan 45°, resulting in failure in achieving perpendicularity. Theobservation also finds that when the off angle is in the range of notless than 59° and not more than 80°, the perpendicularity is improvedand the lasing yield increases to 50% or more. From these experimentalresults, the optimum range of off angle of the GaN substrate is not lessthan 59° and not more than 80°. The same result is also obtained in therange of not less than 100° and not more than 121°, which is an angularrange to provide crystallographically equivalent end faces. The datashown in FIG. 10 is as follows.

Tilt angle, Yield,  0; 10,  0.1; 39,  0.2; 58, 40; 60, 58; 66, 65; 73,70; 75, 65; 79, 60; 85, 35; 90, 30.

The relation between stacking fault density and lasing yield isinvestigated and the result obtained is shown in FIG. 11. The definitionof lasing yield is the same as above. As seen from FIG. 11, the lasingyield is suddenly decreased with the stacking fault density over 1×10⁴(cm⁻¹). The observation of the end face state with an optical microscopeshows that devices in the sample group categorized as decreased lasingyield exhibits the significant unevenness of the end faces, so that noflat fractured faces are obtained. The reason therefor is that adifference in easiness of fracture depends on the existence of stackingfaults. From this result, the stacking fault density in the substrate ispreferably not more than 1×10⁴ (cm⁻¹). The data shown in FIG. 11 is asfollows.

Stacking fault density (cm−1), Yield,  500, 63;  1000, 65;  4000, 60; 8000, 55; 10000, 10; 50000,  1.

The relation between substrate thickness and lasing yield isinvestigated, and the result obtained is shown in FIG. 12. Thedefinition of lasing yield is the same as above. FIG. 12 is a plot forlaser devices in which the stacking fault density of the substrate isnot more than 1×10⁴ (cm⁻¹) and the laser stripe extends along (1) theA-direction. From FIG. 12, the lasing yield is high when the substratethickness is smaller than 100 μm and larger than 50 μm. When thesubstrate thickness is larger than 100 μm, the perpendicularity offractured faces becomes deteriorated. When the thickness is smaller than50 μm, handling of substrates does not become easy and the semiconductorchips become easy to break. From these, the optimum thickness of thesubstrate is in a range of not less than 50 μm and not more than 100 μm.The data shown in FIG. 12 is as follows.

Substrate thickness, Yield,  44, 12;  82, 55;  90, 65; 110, 45; 150, 40;210, 20; 400, 10.

Example 3

In Example 2, the plurality of epitaxial films for the semiconductorlaser are grown on the GaN substrate having the {11-21} surface. Asdescribed above, the end faces for the optical cavity are formed throughthe formation of scribed grooves and by press. In order to findcandidates for these end faces, plane orientations different from thea-plane and making an angle near 90° with respect to the (11-21) planeare obtained by calculation. With reference to FIG. 13, the followingangles and plane orientations have angles near 90° with respect to the(11-21) plane.

Specific plane index, Angle to {20-21} plane, (−1-12 9): 92.77°; (−1-1210): 90.92°; (−1-12 11): 89.37°.The end face may include such an orientation index as above.In the representation of (a1,a2,a3,c), index (−1-12 9) shows a1=−1,a2=−1, a3=2, c=9; index (−1-12 10) shows a1=−1, a2=−1, a3=2, c=10; index(−1-12 11) shows a1=−1, a2=−1, a3=2, c=11.

According to various experiments containing the above-described Examples1 to 3, the angle ALPHA can be in the range of not less than 45° and notmore than 80° or in the range of not less than 100° and not more than135°. In order to improve the oscillating yield, the angle ALPHA can bein the range of not less than 59° and not more than 80° or in the rangeof not less than 100° and not more than 121°. The typical semipolarprimary surface can be any one of the {11-22} plane, {11-21} plane,{11-2-2} plane, and {11-21} plane. Furthermore, the primary surface canbe a slight tilt surface from these semipolar planes.

As described above, the present embodiments provides the group-IIInitride semiconductor laser device with a laser cavity enabling a lowthreshold current, on the semipolar surface of a support base that tiltswith respect to the c-axis toward the a-axis of a hexagonal group-IIInitride. The present embodiments also provides the method of fabricatingthe group-III nitride semiconductor laser device.

Having described and illustrated the principle of the invention in apreferred embodiment thereof, it is appreciated by those having skill inthe art that the invention can be modified in arrangement and detailwithout departing from such principles. We therefore claim allmodifications and variations coming within the spirit and scope of thefollowing claims.

What is claimed is:
 1. A group-III nitride semiconductor laser devicecomprising: a laser structure including a support base and asemiconductor region, the support base comprising a hexagonal group-IIInitride semiconductor and having a semipolar primary surface, and thesemiconductor region being provided on the semipolar primary surface ofthe support base; and an electrode provided on the semiconductor regionof the laser structure, the semiconductor region including a firstcladding layer of a first conductivity type gallium nitride-basedsemiconductor, a second cladding layer of a second conductivity typegallium nitride-based semiconductor, and an active layer, the activelayer being provided between the first cladding layer and the secondcladding layer, the first cladding layer, the second cladding layer, andthe active layer being arranged along a normal axis to the semipolarprimary surface, the active layer including a gallium nitride-basedsemiconductor layer, a c-axis of the hexagonal group-III nitridesemiconductor of the support base tilting at a finite angle ALPHA withrespect to a normal axis toward an a-axis of the hexagonal group-IIInitride semiconductor, the laser structure including first and secondfractured faces, the first and second fractured faces intersecting withan a-n plane defined by the normal axis and the a-axis of the hexagonalgroup-III nitride semiconductor, a laser cavity of the group-III nitridesemiconductor laser device including the first and second fracturedfaces, the laser structure including first and second surfaces and thefirst surface is opposite to the second surface, and each of the firstand second fractured faces extending from an edge of the first surfaceto an edge of the second surface.
 2. The group-III nitride semiconductorlaser device according to claim 1, wherein the angle between the normalaxis and the c-axis of the hexagonal group-III nitride semiconductorfalls within the range of not less than 45° and not more than 80° orwithin the range of not less than 100° and not more than 135°.
 3. Thegroup-III nitride semiconductor laser device according to claim 1,wherein the angle between the normal axis and the c-axis of thehexagonal group-III nitride semiconductor falls within the range of notless than 59° and not more than 80° or within the range of not less than100° and not more than 121°.
 4. The group-III nitride semiconductorlaser device according to claim 1, wherein a thickness of the supportbase is not more than 400 μm.
 5. The group-III nitride semiconductorlaser device according to claim 1, wherein a thickness of the supportbase is not less than 50 μm and not more than 100 μm.
 6. The group-IIInitride semiconductor laser device according to claim 1, wherein laserlight from the active layer is polarized in a direction of an m-axis ofthe hexagonal group-III nitride semiconductor.
 7. The group-III nitridesemiconductor laser device according to claim 1, wherein light in an LEDmode of the group-III nitride semiconductor laser device includes apolarization component I1 in a direction of an m-axis of the hexagonalgroup-III nitride semiconductor, and a polarization component I2 in adirection indicated by a projection of the c-axis of the hexagonalgroup-III nitride semiconductor onto the primary surface, and whereinthe polarization component I1 is greater than the polarization componentI2.
 8. The group-III nitride semiconductor laser device according toclaim 1, wherein the semipolar primary surface is slightly tilted in therange of not less than −4° and not more than +4° with respect to any oneof a {11-22} plane, a {11-21} plane, a {11-2-2} plane, and a {11-2-1}plane.
 9. The group-III nitride semiconductor laser device according toclaim 1, wherein the semipolar primary surface is one of a {11-22}plane, a {11-21} plane, a {11-2-2} plane, and a {11-2-1} plane.
 10. Thegroup-III nitride semiconductor laser device according to claim 1,wherein a stacking fault density of the support base is not more than1×10⁴ cm⁻¹.
 11. The group-III nitride semiconductor laser deviceaccording to claim 1, wherein the support base comprise any one of GaN,AlGaN, AlN, InGaN, and InAlGaN.
 12. The group-III nitride semiconductorlaser device according to claim 1, further comprising a dielectricmultilayer film provided on at least one of the first and secondfractured faces.
 13. The group-III nitride semiconductor laser deviceaccording to claim 1, wherein the active layer includes a light emittingregion provided so as to generate light at a wavelength of not less than360 nm and not more than 600 nm.
 14. The group-III nitride semiconductorlaser device according to claim 1, wherein the active layer includes aquantum well structure provided so as to generate light at a wavelengthof not less than 430 nm and not more than 550 nm.
 15. The group-IIInitride semiconductor laser device according to claim 1, wherein an endface of the support base and an end face of the semiconductor region areexposed in each of the first and second fractured faces, and wherein anangle between the end face of the active layer in the semiconductorregion and a reference plane perpendicular to the a-axis of the supportbase of the hexagonal nitride semiconductor is in the range of not lessthan (ALPHA−5)° and not more than (ALPHA+5)° in a first plane defined bythe c-axis and the a-axis of the group-III nitride semiconductor. 16.The group-III nitride semiconductor laser device according to claim 15,wherein the angle is in the range of not less than −5° and not more than+5° on a second plane perpendicular to the first plane and the normalaxis.
 17. A method of fabricating a group-III nitride semiconductorlaser device, the method comprising the steps of: preparing a substrateof a hexagonal group-III nitride semiconductor, the substrate having asemipolar primary surface; forming a substrate product having a laserstructure, an anode electrode and a cathode electrode, the laserstructure including the substrate and a semiconductor region, and thesemiconductor region being formed on the semipolar primary surface, and;scribing a first surface of the substrate product in part in a directionof the a-axis of the hexagonal group-III nitride semiconductor; andcarrying out breakup of the substrate product by press against a secondsurface of the substrate product, to form another substrate product anda laser bar, the first surface being opposite to the second surface, thesemiconductor region being located between the first surface and thesubstrate, the laser bar having first and second end faces, the firstand second end faces being formed by the breakup, and the first andsecond end faces extending from the first surface to the second surface,the first and second end faces constituting a laser cavity of thegroup-III nitride semiconductor laser device, the anode electrode andthe cathode electrode being formed on the laser structure, thesemiconductor region comprising a first cladding layer of a firstconductivity type gallium nitride-based semiconductor, a second claddinglayer of a second conductivity type gallium nitride-based semiconductorand an active layer, the active layer being provided between the firstcladding layer and the second cladding layer, the first cladding layer,the second cladding layer, and the active layer being arranged along anormal axis to the semipolar primary surface, the active layercomprising a gallium nitride-based semiconductor layer, a c-axis of thehexagonal group-III nitride semiconductor of the substrate tilting at anangle ALPHA with respect to the normal axis toward the a-axis of thehexagonal group-III nitride semiconductor, and the first and second endfaces intersecting with an a-n plane defined by the normal axis and thea-axis of the hexagonal group-III nitride semiconductor.
 18. The methodaccording to claim 17, wherein the angle ALPHA falls within a range ofnot less than 45° and not more than 80° or within the range of not lessthan 100° and not more than 135°.
 19. The method according to claim 17,wherein the angle ALPHA falls within a range of not less than 59° andnot more than 80° or within a range of not less than 100° and not morethan 121°.
 20. The method according to claim 17, wherein the step offorming the substrate product comprises performing processing such asslicing or grinding of the substrate so that a thickness of thesubstrate becomes not more than 400 μm, and wherein the second surfaceis one of the following: a processed surface formed by the processing;and a surface including an electrode formed on the processed surface.21. The method according to claim 17, wherein the step of forming thesubstrate product comprises polishing the substrate so that a thicknessof the substrate becomes not less than 50 μm and not more than 100 μm,and wherein the second surface is one of the following: a polishedsurface formed by the polishing; and a surface including an electrodeformed on the polished surface.
 22. The method according to claim 17,wherein the scribing is carried out using a laser scriber, and whereinthe scribing forms a scribed groove, and a length of the scribed grooveis shorter than a length of an intersecting line between the firstsurface and an m-n plane defined by the normal axis and an m-axis of thehexagonal group-III nitride semiconductor.
 23. The method according toclaim 17, wherein the semipolar primary surface is any one of a {11-22}plane, a {11-21} plane, a {11-2-2} plane, and a {11-2-1} plane.
 24. Themethod according to claim 17, wherein an end face of the active layer ineach of the first and second end faces makes an angle in the range ofnot less than (ALPHA−5)° and not more than (ALPHA+5)° with respect to areference plane in a plane defined by the c-axis and the a-axis of thehexagonal group-III nitride semiconductor, the reference plane beingperpendicular to the a-axis of the support base of the hexagonal nitridesemiconductor.
 25. The method according to claim 17, wherein thesubstrate comprises any one of GaN, AlGaN, AlN, InGaN, and InAlGaN.